In a common sewing machine, a sewing thread is usually cut via a thread cutting mechanism when coming to an end of stitches, so as to prepare for a next round of sewing. For example, the Taiwan Patent No. M383002, “Thread Trimming Structure for 3-Needle Coverstitch Machine with Extending Arm”, discloses a thread trimming structure mounted on a sewing machine. The thread trimming structure includes a signal-controlled driver set for driving the thread trimming structure to operate. The driver set drives a pushing rod by a power source to perform a back-and-forth movement, i.e., to extend forward and return to an original position, so as to cause the pushing rod to drive a linkage set to actuate a hook element of a hook and cut set. As such, without manually trimming a thread, an end of the thread of a workpiece is withdrawn through hooking and then trimmed by a trimming blade, thereby achieving automatic thread trimming. For another example, the Taiwan Patent No. I320437 discloses a thread cutting device for a sewing machine. The thread cutting device, connected to a body of a sewing machine, includes a cam mechanism, a retaining mechanism, a supporting mechanism, a scissor driver set and thread scissors. The cam mechanism includes a cam, scissors and a positioning cam arm. The retaining mechanism includes a retaining arm and a movable assembly capable of sliding relative to the retaining arm. The supporting mechanism includes a base plate driven by the cam arm and connected to the movable assembly, and a supporting arm set connected to the base plate. The scissor driver set includes a driving arm connected to the scissor cam arm and a lever pivotally connected to the supporting arm set. The thread scissors include a fixed blade connected to the supporting arm set and a movable blade connected to the fixed blade.
In the thread cutting mechanisms for a sewing machine in the above disclosures, a structure for driving a blade is operated via a connecting rod set having at least two connecting rods. As a result, not only the structure is made more complicated, but also thread cutting operation may not be precisely achieved due to errors caused by size differences or disengagement of the connecting rods during a transmission process. Further, a conventional thread cutting mechanism is driven by a positioning motor that controls rotation of a cam, and a thread cutting speed of the scissor driver set, instead of being directly controlled by the positioning motor, is controlled through controlling a rotational speed of the cam by the positioning motor. That is to say an indirect drive mechanism is employed as the cam is provided between the scissor driver set and the positioning motor, thus leading to a greater possibility in imprecisely controlling thread cutting operation as described above.